Torsional vibrations are the rotational irregularities of a rotatingly driven component. In a vehicle drivetrain, torsional vibrations are caused by the forces generated within a combustion engine by the combustion of gases during the periodic combustion process. Torsional vibrations of the second or third order which originate from the engine, as a result of the ignition frequency of four or six cylinder engines, respectively, are predominant in the vehicle driveline. Torsional vibrations not only emanate from the engine power pulses but also from torque spikes and from abrupt changes in driveline torque due to rapid engine acceleration and deceleration.
Torsional vibrations cause premature wear to driveline components as well as audible noise. In a conventional driveline, the flywheel, which is rigidly connected to the crankshaft, will generate high reaction forces on the crankshaft. Torque irregularities from a periodic combustion also engine adds additional stress in the form of high frequency torques to the transmission. Furthermore, when a manual transmission is in neutral, gear rattle occurs, which is also an audible event, due to the teeth of meshing gears lifting away from another and then striking each other as a result of high frequency torque fluctuations.
Along with gear rattle, order based responses from the second or third engine order may be passed through the drivetrain and into the body structure. This sound can be greatly amplified if the components forming the sound are excited at their resonant frequencies.
Torsional vibration issues are further compounded by efforts to improve vehicle efficiency. Reductions in vehicle size and weight as well as reductions in driveline component inertia, such as flywheel masses, as well as reductions in transmission oil viscosity have added to the existing torsional vibration challenges. Lower drivetrain inertia results in a higher natural frequency of the drivetrain. As the engine rotational speed passes through the drivetrain natural frequency, resonant frequency occurs. The input displacement of a system is amplified at resonant frequency.
It is well known in the art to incorporate torsional vibration damping mechanisms in a dry clutch. As rotation occurs, the energy storage means within the damper, typically coil springs, provide the rotational compliance between the rotating elements. Another component of the damper is hysteresis, which is provided by friction producing elements. The hysteresis cooperates with the energy storage component of the damper to remove energy from the system. The prior art is replete with friction clutches with dampers for attenuating torsional vibrations. A variety of spring arrangements have been employed to provide frictional force for damping.
The coil springs are typically disposed in spring pockets circumferentially located around a clutch hub. Compression of the springs is typically limited by a stop disposed between the hub and the disc limiting relative rotation therebetween. The springs provide some isolation between the engine and transmission of firing pulses of the engine and other engine speed fluctuations. However, point loading between the springs and the spring pockets occurs at ends of the springs, producing wear of one or both parts. Also, within the range of travel permitted by the stop or stops, the springs tend to move relatively freely within the pockets, bowing and rubbing against the sides of the pockets. This spring motion and wear may potentially lead to the springs breaking, or to an increase in the size of the spring pocket, diminishing the dampening effectiveness of the clutch and potentially enabling the spring to escape the clutch driven disc assembly.
It is desired to minimize such wear and the associated loss of damper effectiveness by improving the interface between the components to increase the effective life of the dampers.